1. Field of the Invention
The present invention relates to a contact detecting apparatus for detecting the contacting of a tool with a workpiece when the tool and the workpiece are moved relatively with respect to one another.
2. Description of the Prior Art
A prior contact detecting apparatus contains a coil mounted on the end of a spindle head and connected to an AC power source. When a tool is moved into contact with a workpiece, the output signal of the coil is caused to change, since a looped circuit connecting a spindle, the tool, the workpiece and a bed is completed. Therefore, the change of the output is used to detect that the tool has made contact with a workpiece.
In a conventional machine tool having this feature, a contact detecting signal from the contact detecting apparatus is used to cause the machine tool to change its feed rate from a rapid free rate to a machining feed rate and to measure the distance between the reference position and the workpiece. However, when the contact detecting apparatus fails to indicate the contact for some reasons, such as malfunction of the contact detecting apparatus or breaking down of the wire of the coil, damage may be caused not only to the workpiece but also to the tool because the feed rate of the machine tool is not altered to the machining feed rate and/or the tool is not stopped even after making contact with the workpiece.